This section provides background information related to the present disclosure which is not necessarily prior art.
When launching an explosive device on a trajectory towards a given target, the device can be made safer by equipping it with a safing system. The safing system prevents the device from being armed until after it has traveled a safe distance from the launch site. The safing technique has been traditionally employed in artillery shells, for example, by utilizing a mechanical system that counts the number of spiral rotations the shell makes in flight. In this regard, the spiral rifling pattern within the bore of the cannon imparts the spiral rotation to the shell. Thus, knowing the muzzle velocity of the shell and the geometry of the rifling pattern, one can calculate how many shell rotations will take place by the time the shell reaches a safe distance from the launch site. The mechanical system simply counts those rotations and arms the device after the safe number of rotations has occurred. Torpedoes launched from submarines work in a similar fashion, by counting the number of rotations of the torpedo's propeller after launch.
This counting-rotations technique is not applicable to all types of devices, however. For example, self-guided missiles, dropped bombshells and other projectiles may be launched or deployed without a spiral rotation imparted. In such devices there is no reliable spiral rotation to count; thus conventional rotation counting safing systems do not work. One traditional solution in such cases has been to use a single-axis accelerometer located on the device, which by sensing motion in the launch direction can provide a signal indicative of distance from the launch site. If the trajectory of the device follows a predictable, known path, such as a parabolic arc induced by forces of gravity, a single-axis accelerometer can provide a useful distance measure. However, if the device deviates from the predictable known path for some reason, the single-axis accelerometer may be of limited value and a safing system based on a single-axis solution cannot be relied upon in all cases. A single-axis solution would not be able to accurately assess the safe distance if the device is a self-guided missile that has made a U-turn and has doubled back on its trajectory, for example.